This invention relates to a halftone generation system and a halftone generation method for electronically converting multilevel (continuous-tone) image data into binary image data and in particular to a halftone generation system and a halftone generation method for supplying halftone data to a high-definition print processing system at high speed and a high-speed color print processing system of high image quality at high speed.
A print processing system for printing a color image generally is separated into four plates of C (cyan), M (magenta), Y (yellow), and K (black) and multilevel image data is stored on the color plates. The multilevel image data on each color plate is compared with threshold matrix data and is converted into binary image data. Halftone printing is performed based on the binary image data for each color and a color image is generated.
To form a halftone image digitally in conventional offset printing, etc., it is a common practice to form a halftone image on a photosensitive film with a high-resolution laser recorder called an image setter, then print it on a PS (presensitized plate), etc. In the image setter technique, halftone formation is executed offline as mentioned above and a high speed is not required for halftone formation, thus generally halftone formation is handled using a CPU (central processing unit) contained in the image setter. That is, a comparison operation is performed on multilevel image data and threshold matrix data by the CPU and the result is recorded in memory in the image setter. Upon completion of halftone formation in page units or job units, the halftone data recorded in the memory is read in sequence and is recorded on a photosensitive film in a laser.
On the other hand, in electrophotographic print processing systems, it is also becoming a common practice to digitally form a halftone image with electronization of information and colorization and high resolution of recorders in recent years. In the electrophotograpy, unlike general printing mentioned above, it is necessary to form a halftone image-continuously from multilevel image data matching output of a recorder. Thus, it is a common practice to perform hardware processing rather than CPU processing with respect to halftone formation of electrophotographic print processing systems.
FIG. 2 shows the hardware configuration for representative halftone formation in a related art. In FIG. 2, an address generation section 8p calculates an address of threshold matrix data for comparison in response to the pixel position of input-multiple-valued image data 1p and outputs the address to threshold matrix data memory 2′p storing threshold matrix data. From the threshold matrix data memory 2′p, threshold data 9p corresponding to the input multiple-valued image data is output to a comparator 6′p, which then compares the input multiple-valued image data with the threshold data and outputs halftone data as binary image data 7′p. 
As the threshold matrix data for the electrophotographic print processing systems mentioned above, a technique called rational tangent is generally used and threshold data is formed of a comparatively small matrix; the flexibility of the angle and the number of lines of each color plate for color printing is comparatively small. However, also in the electrophotographic print processing system, it has been made possible to form a halftone according to a supercell technique or a multiunit area technique wherein the matrix size formerly used with an image setter is large and the flexibility of the angle and the number of lines is high with a recorder put into high resolution as high image quality has been required in recent years.
In the halftone formation in FIG. 2, it is common practice to generate multilevel image data in print processing of the print processing system in the related art and convert the multilevel image data into halftone data just before laser exposure in a recorder; however, the costs of memory for storing multilevel image data introduce a problem in a low-priced electrophotographic print processing system as compared with the image setter as the recorder is put into high-resolution. Further, fast transfer of a large amount of multilevel image data from a print processing section to a recorder and fast halftone data generation in response to the recording speed of a fast recorder also introduce problems.
Converting multilevel image data into halftone data in the print processing section is designed as one resolution means to the first two problems. That is, multilevel image data is previously converted into halftone data and the halftone data is stored in memory as binary data and is output to the recorder, whereby both the memory capacity and the data transfer rate are reduced to one eighth. However, the last problem, fast halftone data generation, remains unsolved.
A document disclosing a technology of generating halftone data at high speed is the Unexamined Japanese Patent Application Publication No. Hei 6-6606, which describes the technology wherein one line of threshold matrix data is transferred a plurality of high-speed memories alternately apart from threshold matrix data memory and halftone processing is executed by reading the threshold data from the high-speed memories. However, the Unexamined Japanese Patent Application Publication No. Hei 6-6606 aims at execution of halftone generation at high speed with high-speed memories of a small capacity; basically it is the same as the configuration shown in FIG. 2 and dramatic speeding up of processing cannot be expected.
Particularly, for painting objects with an average drawing run length for each painting object such as text or graphics, access to low-speed memory storing the whole threshold matrix data occurs frequently and benefit from high-speed memory can be little received.
Another idea for generating halftone data at high speed is as follows: A plurality of pieces of halftone data generation hardware as shown in FIG. 2 are provided and are operated in parallel, thereby executing halftone processing. In such a parallel operation technique, a large-scaled and high-speed circuit is provided for a logical operation section such as a comparator owing to the recent advance of the ASIC technology and the logical operation section can be realized easily. However, reading of threshold data involves problems such that generally the memory access time is slow as compared with simple logical operation performed as in a comparator and that a large number of data lines for reading a plurality of threshold data pieces at the same time are required; speeding up of reading threshold data is not achieved, causing a bottleneck in the technique.
It is therefore an object of the invention to provide an art of enabling high-speed threshold data output otherwise causing a bottleneck in hardware for generating a plurality of halftone data pieces.
Previous applications of using a crossbar switch to sort image data in an image processing system include the Unexamined Japanese Patent Application Publication No. Hei 8-305839, etc. The method shown in the Unexamined Japanese Patent Application Publication No. Hei 8-305839 is as follows: As 16-bit image data for one color per pixel, the high-order eight bits of each of R, G, B, and dummy (D) are first stored at consecutive addresses of memory, then the low-order eight bits of each of R, G, B, and D are stored at consecutive addresses following those addresses and to transfer the data to a display device, only the high-order eight bits at a time are transferred, thereby transferring the data at high speed. A crossbar switch is used to sort image data when image processing is performed by a CPU. However, the method disclosed in the Unexamined Japanese Patent Application Publication No. Hei 8-305839 is specialized for display and assumes only two ways of using the crossbar switch; it cannot be applied to such a halftone generation system incorporating the invention.
Previous applications of using a barrel shifter to sort image data in an image processing system include the Unexamined Japanese Patent Application Publication Hei 9-247466, etc. In the Unexamined Japanese Patent Application Publication No. Hei 9-247466, a barrel shifter, a selector, a register, and the like are used to realize insertion of special code and its relevant operation when variable-length code data is concatenated in a coder for performing variable-length coding, thereby speeding up processing by hardware. However, the method disclosed in the Unexamined Japanese Patent Application Publication No. Hei 9-247466 is specialized for the coder for performing variable-length coding; it cannot be applied to such a halftone generation system incorporating the invention.
It is therefore another object of the invention to provide a halftone generation system and a halftone generation method capable of executing at high speed, threshold data read and threshold data sort processing otherwise causing a bottleneck in hardware for generating a plurality of halftone data pieces in parallel.